Oxygen is required in increasingly greater quantities in industrial combustion and oxidation processes. Oxygen is typically separated from air by processes such as cryogenic distillation, adsorptive separation, chemical absorption and differential permeation through membrane media. Solid electrolyte membranes (SEMs) are currently being studied as an alternative technology for separating oxygen from oxygen-containing gaseous mixtures such as air. The most promising SEMs are formed from multicomponent metallic oxides such as titania-doped yttria-stabilized zirconia (YSZ) and praseodymia-modified zirconia.
Gas separation membranes formed from multicomponent metallic oxides are typically operated at high temperatures (e.g. 700.degree. C. or more) wherein the membranes conduct both oxygen ions and electrons and can thereby operate without the encumbrance of external circuitry such as electrodes, interconnects and power supplies. When a difference in oxygen partial pressure exists on opposite sides of the multicomponent metallic oxide membrane and operating conditions are properly controlled, pure oxygen is produced as oxygen ions migrate to the low pressure side of the membrane while an electron flux occurs in the opposite direction in order to conserve charge.
Several laboratories are investigating methods for preparing thin films of multicomponent metallic oxides suitable for use as inorganic membranes. However, currently available preparative methods for making such thin films are not totally satisfactory. For example, stoichiometry of the multicomponent metallic oxide is sometimes difficult to control and detrimental chemical reactions sometimes occur between the multicomponent metallic oxide and the substrate due to the high temperatures required to form the required layer.
Prior art methods for making multicomponent metallic oxides include U.S. Pat. No. 4,330,663 which discloses SEMs formed from a sintered body of a multicomponent metallic oxide comprising an oxide of cobalt, an oxide of at least one metal selected from strontium and lanthanum and an oxide of at least one metal selected from bismuth and cerium. The multicomponent metallic oxides are prepared by a time-consuming method wherein lanthanum oxide, bismuth oxide and cobalt acetate are ground, fired at high temperatures, reground and sintered.
Japanese Patent Application No. 61-21717 discloses SEMs which are formed from multicomponent metallic oxides represented by the formula La.sub.1-x Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3-d wherein x ranges from 0.1 to 1.0, y ranges from 0.05 to 1.0 and d ranges from 0.5 to 0. The mixed oxides are prepared by heating a mixture of hydroxide precipitates formed by adding an alkaline solution such as ammonia water or caustic alkali to a solution of the metal salts. The mixture is heated at a temperature between 650.degree. C. and 1400.degree. C., preferably between 850.degree. and 1260.degree. C., for a few hours or dozens of hours depending on the temperature, and then sintered at temperatures between 1000.degree. C. and 1500.degree. C., preferably between 1150.degree. and 1350.degree. C.
Teraoka and coworkers, Nippon Seramikkusu Kyobai Gabujutsu Ronbushi, 97 (1989) 533 disclose various methods for depositing a thin La.sub.0.6 Sr.sub.0.4 CoO.sub.3 perovskite multicomponent metallic oxide layer onto a porous support of the same material by an rf sputtering technique and a liquid suspension spray deposition method. The film produced by the rf sputtering method was cracked and porous. While the liquid suspension spray deposition technique produced a film approximately 15 .mu.m thick following sintering at 1400.degree. C. and repeated deposition/sintering cycles, the film showed unexpectedly low oxygen permeability due in part to the extremely high temperatures required to produce the desired non-porous morphology.
Y. S. Lin, in his Doctoral Thesis entitled, "Chemical and Electrochemical Vapour Deposition of Zirconia-Yttria solid Solutions in Porous Ceramic Media", Universiteit Twente, The Netherlands, defended on Jan. 3, 1992, teaches oxygen transport membranes which are prepared by electrochemical vapor deposition (ECVD) from mixtures of metal halides and water (oxygen). ECVD is a high temperature route (&gt;800.degree. C.) which evolves hydrogen halides, and operates with the reactants in opposition, and combines a chemical vapor infiltration (CVI) step with subsequent film growth by electrochemical transport of oxygen through the deposited layer.
U.S. Pat. No. 4,609,562 discloses an ECVD apparatus and procedure for fabricating solid oxide fuel cells. The patent states that the pressure drop across the tubular substrate must be controlled during the initial CVI step of the ECVD process. The CVI reaction was completed at 1200.degree. C., using a 5-10 Torr pressure differential across the tube wall. A reactant mixture of YCl.sub.3 /ZrCl.sub.4 /argon and an oxidant composed of oxygen(20%)/water(40%)/carrier gas(40%) was employed wherein the oxidant was forced to flow through the walls of the porous tubular support. Pore closure and was indicated by a pressure rise in the tube bore which corresponded to the oxidant feed side of the tube.
An improved method is being sought for depositing a layer of a multicomponent metallic oxide onto a porous support while overcoming problems associated with membrane leakage cased by residual porosity with in the membrane. Desirably, such a method would utilize lower temperatures than previously utilized in order to promote formation of low stress coatings, to reduce chemical reactions between the multicomponent metallic oxide layer and porous support and to minimize changes in the substrate microstructure caused by heating the material at unduly high temperatures.